It is well known that quantities of vapor tend to remain undissolved in aircraft fuel while the fuel is stored, for example, in the fuel tank of an aircraft at ground level. It is also well known that such dissolved vapor may separate from the fuel, and become undissolved free vapor within the physical confines of the system, when the ambient pressure decreases for example as the altitude of the aircraft increases during flight. Other factors, such as ambient temperatures and the kinetic energy imparted to the fuel in the course of mechanical displacement within the aircraft fuel system, also tend to affect the separation of vapor from the liquid fuel.
The design and physical size requirements of commercial and military aircraft often result in the fuel tank or tanks being positioned relatively far away from the engine. For example, in combat helicopters, it is common for the fuel tank to be located perhaps six or seven feet below the engine, measured in the vertical direction, and perhaps an equal number of feet away from the engine in the horizontal direction. In this context, the terms "vertical" and "horizontal" refer generally to the customary gravitational and earth-bound horizon alignment axes, respectively. The fuel conduits, generally pipes and tubes, that carry fuel from the tank to the engine, necessarily include various vertical and horizontal portions to accommodate and conform to the structural frame of the aircraft. It has been found that substantial quantities of free vapor can accumulate easily and unintentionally in the portions of the fuel system conduits that include significant changes in direction, such as right angle bends and "ell" couplings.
At any point where the upper end of a vertical section of a fuel conduit forms a right angle connection with a horizontal section, the possibility exists that the right angle may assume a position where it becomes the vertex of an inverted "v", depending upon the flight "attitude" or position of the aircraft. When this occurs, free vapor that is mixed with fuel traveling through the fuel conduit tends to accumulate at this vertex. As a result, the less dense, lighter vapor tends to rise above the heavier fuel and becomes trapped within the confines of the downward sloping conduit sections on either side of the bend. If an aircraft continues in this position for a long enough period of time, which may be merely a matter of several seconds, depending upon the nature of the aircraft and the fuel system involved, the volume of vapor that accumulates in such an inverted "trap" may be substantial (e.g., a vapor "bubble" may fully occupy a continuous length of four feet or more of a fuel conduit). It must be understood that the vapor accumulated in this manner will not necessarily interrupt the flow of liquid fuel through the bend in the conduit since the fuel may continue to flow through the bend, albeit in a constricted manner, between the trapped vapor bubble and the lowest interior surface of the conduit.
However, in the event of a subsequent abrupt change of position, the accumulated vapor may be released suddenly into the fluid flow path as the horizontal portion of the conduit returns to its normal position or becomes tilted upwardly relative to the right angle bend. The fluid "bubble" thus released, representing a volume from which fluid fuel is entirely excluded, may have a length of several feet in ordinary fuel supply systems of known designs. The bubble will be carried through the fuel system conduit to the engine, assuming the bubble does not adversely affect the pumping ability of the pump so as to completely prevent the displacement of fuel through the system as discussed more fully hereinbelow. When the continuous supply of combustible liquid fuel is interrupted by the discharge of a vapor bubble, having a significantly lower level of combustibility, into the combustion chamber of an engine, the combustion process is undesirably and unintentionally affected; engine power may be significantly reduced, or the combustion process mail be at least temporarily terminated entirely, an occurrence commonly referred to as "flame-out". During flame-out, the engine is unable to provide adequate power to the aircraft, thereby adversely affecting the flight of the aircraft while placing the aircraft and personnel in extreme danger. In addition, restoration of normal engine operation often requires the pilot to rapidly complete a series of separate engine control tasks.
A disadvantage of many existing fuel supply systems for aircraft is the inability to quickly and effectively re-establish fuel flow to the engine once flame-out has occurred. All fuel supply systems require at least one fuel pump to deliver fuel from the fuel supply tanks to the engines. As a result of aircraft design requirements, once flame-out occurs, it is often necessary for the pump to draw or raise liquid vertically through a distance of several feet from the supply tank, through an at least partially empty conduit. This capability is known as "self-priming". Positive displacement pumps, such as gear pumps, are known to have this capability and are widely used in aircraft for delivering fuel directly to the engine. However, it is also well-known that positive displacement pumps require a certain minimum level of inlet pressure to assure that fuel does not vaporize as it enters the pump. Such vaporization can cause a substantial and adverse reduction in pump output and overall performance. One condition that is known to result in low inlet pressure is a relatively long vertical rise between the pump inlet and the supply tank from which the fuel is being pumped.
To overcome the undesirable low pressure at the inlet of a "main-stage" positive displacement type fuel pump, a "boost pump" may be provided in the main fuel path between the supply tank and the inlet of the main stage pump. The purpose of the boost pump is to raise the ambient pressure of the fuel in the supply line upstream of the main stage pump to a level that will satisfy the minimum inlet pressure required at the fuel inlet of the main stage pump. Centrifugal pumps or impeller pumps are known to be particularly well suited to supplying acceptable quantities of liquid at relatively constant pressure independently of the inlet fluid pressure. Accordingly, centrifugal pumps are commonly employed in aircraft fuel systems as boost pumps to provide a steady supply of liquid fuel at relatively constant pressure to the inlet of a positive displacement main stage pump.
However, centrifugal pumps are also subject to certain operating restrictions which tend to make them less than satisfactory for use as boost pumps. For example, the basic design of a centrifugal or impeller pump generally cannot produce sufficient vacuum at its inlet to achieve self-priming operation. As a result, when a large vapor bubble enters the inlet of a conventional centrifugal type boost pump, an interruption of the inlet fuel supply occurs, often resulting in a complete loss of the "prime" condition of the boost pump. As a result, the boost pump is incapable of displacing fuel through the supply conduit thereby terminating the supply of fuel to the engine, thus resulting in engine flame-out. Moreover, quick and effective re-priming of conventional centrifugal boost pumps used in aircraft fuel systems is practically impossible. Consequently, this type of extended flame-out occurrence results in extreme danger to the pilot and aircraft.
To help avoid the risk and consequences of such a fuel shutdown, modified forms of centrifugal pumps have been developed that provide both satisfactory self-priming capability and relatively constant output pressure capability. Such pumps are known generally as side-channel pumps. They are characterized by the ability to cause the liquid fuel to re-absorb undissolved fuel vapor to process large quantities of such vapor. As a result, side-channel pumps automatically reprime the system following a flame-out and rapidly restore the supply of liquid fuel so as to permit restoration of normal engine operation. It should be understood that the delivery of large amounts of undissolved vapor to the side-channel pump still may cause temporary periods of flame-out if the amount of vapor exceeds the pump capability. These side-channel pumps do not decrease the amount of undissolved vapor delivered to the engine and, therefore, do not prevent or even minimize the occurrence of flame-out.